1. Field of the Invention
The present invention relates to an image forming process and apparatus, and particularly to a recording process by which the back of a photoconductor is exposed to light for almost simultaneous development, in order to achieve improved image quality.
2. Description of the Related Art
Current copying machines and page printers generally employ an electrophotographic recording system. This system uses a photoconductor as the recording medium, and usually involves the Carlson process by which recording is accomplished through 7 steps including uniform charging, image exposure, development, transfer, fixing, static elimination and cleaning. The charging produces a uniform positive or negative static charge on the surface of a photoconductive photoconductor, and in the subsequent exposure process laser light, etc. is irradiated to remove the surface charge on specific sections, thus forming on the photoconductor a static latent image corresponding to the image information. Next, the latent image is electrostatically developed with toner, to form a visible image on the photoconductor with the toner, and finally the toner image is electrostatically transferred to recording paper and fused therewith by heat, light, pressure, etc. to obtain a print. However, in conventional recording apparatuses which employ the Carlson process, the means used for each step are positioned around the photoconductor, and therefore when the apparatus is downsized the means for each step become closely arranged around the photoconductor. This has resulted in the disadvantages that downsizing may be accomplished only to a limited degree and that scattering of the developing agent from the developing device contaminates the optical system used in the image exposure means, thus adversely affecting the printing.
In light of these disadvantages there has recently been proposed an image exposure process in which the image exposure light source is located inside the photoconductor, and light is irradiated from behind the photoconductor opposite the developing device, to complete recording in the width of contact between the developing agent and the photoconductor (for example, Japanese Unexamined Patent Publication No. 58-15395 and No. 57-119375).
This principle will now be explained.
FIG. 1 shows the construction of an apparatus for the process described above. The photoconductor 1 is composed of a transparent substrate 1a, a transparent conductive layer 1b and a photoconductive layer 1c, and the transparent conductive layer is grounded. The developing roller is provided with a conductive sleeve on an anchored magnetic roller. A low resistance carrier and insulative toner are used as the developing agent 4. The developing agent is attracted to the developing roller by magnetic force, and is carried to the photoconductor while adhering to the surface of the sleeve. The following three steps are then carried out inside the developing nip in rapid succession. The photoconductor is charged with the developing agent by the bias voltage applied to the developing roller. Image exposure is accomplished on the charged photoconductor from the transparent substrate side to form a latent image. The toner is developed on the exposed sections by inverted development. The developed toner is then transferred to recording paper 5 or a plastic plate to obtain a print. Hereunder this process will be referred to as the "back-light process", because the photoconductor is exposed to light from the back side.
The back-light process is based on a simple image forming principle, and the low number of parts makes it very effective for downsizing of the apparatus.
The back-light process, however, produces lower image quality than the Carlson process, and of particular consideration is the tendency toward staining of the print background, known as fogging.
In particular, since the developing principle of the back-light process inevitably causes the surface potential of the photoconductor drum to be either the same as the developing bias or slightly lower due to efficiency loss, toner tends to adhere to the non-image sections outside the latent image, causing fogging, and such development is even more likely in a low-humidity environment in which the toner charge amount is higher.
In order to reduce fogging the toner charge amount must be kept low, and thus Japanese Unexamined Patent Publication No. 59-88751 discloses a back-light process employing conductive toner. Since the toner is conductive, the surface potential of the photoconductor drum is lowered in the non-image sections, and since the toner charge amount is lowered, the recovery is improved.
Nevertheless, the conductivity of the toner in this process creates the disadvantage of lower transfer efficiency in corona or roller systems.
Thus, the present invention has been accomplished using an insulative toner with high transfer efficiency to reduce fogging.
Numerous processes have already been reported wherein insulative toner is used for back-light development.
One process, proposed in Japanese Unexamined Patent Publication No. 61-46961, deals with the problem from the apparatus end by employing two electrodes, a charging bias electrode and a developing bias electrode. This, however, complicates the construction of the apparatus, and thus constitutes an obstacle to downsizing and simplification, which were the original advantages of the back-light process.
On the other hand, as a solution from the carrier end, there is disclosed, in Japanese Unexamined Patent Publication No. 5-119541 and elsewhere, a process whereby the developing agent resistance is lowered by conductive treatment of the carrier surface, in order to raise the surface potential of the photoconductor drum and thus reduce fogging. Since the photoconductor drum is charged via the developing agent in the back-light process, lowering the resistance of the carrier is highly effective for achieving higher picture quality. However, when using a developing agent which is mixed with the insulative toner at a high toner concentration, the resistance of the developing agent eventually increases, and thus no effect is obtained.